As it says in the description of the Thermaltake ATX splitter cable, who also makes PSU's.
https://www.amazon.com/Thermaltake-24-Pin-Mining-Adapter-AC-005-CNONAN-P1/dp/B00O0M6Q0CCombine your power needs with the Thermaltake Dual 24pin Adapter Cable. Ideal for crypto currency system builds and extreme gaming, optimize your power with simultaneous on/off functionality to extend your GPU performance. With independent power supplies, take your system further for more mining and ability to meet the needs of multi-GPU power requirements. PLEASE DO NOT MIX POWER CONNECTIONS TO MOTHERBOARD.
You should never mix power supplies to devices connected to the motherboard. USB risers are not electrically isolated from the motherboard and therefore it's important that all the risers are powered by the same PSU that powers the motherboard.
I also noticed BBT in some of his his latest live streams emphatically recommends that for a dual PSU rig, you need to power the risers with the same PSU that powers the motherboard and only use the second PSU to power the GPU's.
https://www.youtube.com/watch?v=lXZondkAWVc&feature=youtu.be&t=1253https://youtu.be/pQ-EAunoAqY?t=988It's true that you should never mix connections to the motherboard, but that's not at all what is happening with USB risers. USB risers do not send power to the motherboard - they send it to the video card. The only thing transferred between the riser and the motherboard is data. Yes, it's using electricity to do that, but at very low levels - for enough power to be sent over that connection to cause damage to hardware, something else would have had to catastrophically fail.
Mixing PSUs in general is a really, really bad idea, but if you have to do it, each GPU + Riser should be considered a whole unit. Voltage imbalance between two sources can fry whatever is in between. The video card is what is in between the sources, not the motherboard.
I tested the outer pins of the USB cable with a multimeter. There was 3.3V coming from the motherboard to the the connector on the riser and there is also a ground connection between the riser and the motherboard. There is no problem with mixing power supplies if done correctly. As explained by BBT and the link I posted in my original post, the problem is not the amount of current, it's mixing grounds from multiple sources. Which is why you MUST power all the risers with the same PSU that powers the motherboard to avoid ground loops.